Embodiments of the present invention relate to gas turbines in mechanical drive applications. More specifically, the subject matter disclosed herein concerns multiple-shaft gas turbines, such as aeroderivative twin-shaft gas turbines for mechanical drive applications.
Gas turbines have found a wide use in several applications, such as power generation, as well as mechanical drive, where the gas turbines are commonly used as first mover for one or a plurality of driven machines, such as compressors, in particular, centrifugal compressors. Typical mechanical drive applications are in the field of natural gas liquefaction, carbon dioxide recovery and the like.
A gas turbine includes one or more sequentially arranged air compressors for compressing ambient air, a combustor burning fuel together with the compressed air and one or more turbines for driving the compressor(s) and generate useful mechanical power. The power generated by the turbine(s) exceeding that required to drive the compressor(s) is used for driving the load.
Gas turbines ingest a large amount of air. Particles in the form of aerosols present in the air sucked by the gas turbine partly exit the gas turbine with the exhaust gases. However, there are particles which contaminate the turbomachinery, by sticking to the stationary vanes and to the rotary blades thereof. This contamination, also called fouling, particularly negatively affects the initial part of the flow path within the gas turbine, i.e. the compressor or compressors. Contaminants forming deposits on the stationary blades (vanes) and rotary blades of the compressor alter the geometry of the blades and increase gas friction, thus reducing the overall compressor efficiency. In particular, the particles stuck to the surfaces of the vanes and the rotary blades of the compressor alter the aerodynamic properties of the flow passages defined by the blades and the vanes. The alteration of the aerodynamic properties causes loss of mass flow and therefore reduction of the compressor efficiency.
Typically the compressor of a gas turbine consumes the major part of the power generated by the turbine or turbines, i.e. approximately 60% of said power. A reduction in the compressor efficiency thus negatively affects the overall efficiency of the gas turbine, reducing the power available for driving the load.
One of the ways to reduce fouling of the compressor in a gas turbine is to wash the gas paths in the gas turbine. Washing is typically practiced by injecting a washing liquid in the gas path upstream of the compressor inlet. The turbomachinery is allowed to rotate during washing such that the liquid is forced through the compressor and exits at the rear of the gas turbine. The wash liquid can contain water and chemical additives and is fed in the form of a fine spray which will distribute the washing liquid over the entire compressor inlet face. Atomization is provided by suitable nozzles which are fed with pressurized washing liquid.
An effective way of washing the gas turbine is the so-called offline washing. In this case washing is performed while the gas turbine is not fired, but is turning at a rotary speed which is a fraction of the rated rotary speed during normal operation, i.e. when running at load. An additional mover is required, to keep the gas turbine rolling at offline washing speed.
Online washing is also possible. In this case the gas turbine is washed while running under load conditions. Such washing process is, however, less effective, due to the speed and temperature conditions in the compressor, which result in inefficient washing of the blades, centrifugation of the washing liquid towards the casing of the compressor and evaporation of the washing liquid due to the temperature increase provoked by the high compression ratio. When online washing is used, fouling of the compressor can only be reduced but not avoided. Therefore, offline washing capability must anyhow be available. The gas turbine will in fact require offline washing when the amount of particle deposits on the vanes and rotary blades of the compressor becomes unacceptable, in spite of online washing.
Aeroderivative gas turbines are increasingly used for machine drive and power generation applications. Some aeroderivative gas turbines comprise a multi-shaft arrangement. A multi-shaft arrangement is one in which more than one shaft is provided, to drivingly connect turbines and compressors to one another. In some multi-shaft gas turbines, the power turbine, i.e. the turbine which provides the mechanical power to drive the load, is mechanically connected through one of the gas turbine shafts with one of the compressors.
FIG. 1 shows a schematic diagram of a twin-shaft aeroderivative gas turbine used in a typical mechanical drive application. Reference number 1 globally indicates an apparatus comprising a gas turbine and a load. The gas turbine 3 comprises a low pressure compressor 5, a high pressure compressor 7, a high pressure turbine 9 and a low pressure turbine, or power turbine 11. The high pressure compressor 7 is drivingly connected to the high pressure turbine 9 by means of a first gas turbine shaft 13. The low pressure turbine or power turbine 11 is drivingly connected to the low pressure compressor 5 by means of a second gas turbine shaft 15, arranged coaxial with the first gas turbine shaft 13 as well as coaxial with the high pressure turbine 9 and the high pressure compressor 7.
Ambient air is compressed by the low pressure compressor 5 and by the high pressure compressor 7 and enters a combustor 17 where gaseous or liquid fuel is added to the compressed air stream and burned to generate a flow of high-pressure, high-temperature combustion gases. The combustion gases are sequentially expanded in the high pressure turbine 9 and in the low pressure turbine 11 before being discharged.
The power generated by the expansion of the combustion gases in the high-pressure turbine 9 is entirely exploited to drive the high pressure compressor 7. Conversely, the mechanical power generated by expanding the combustion gases in the low pressure turbine 11 is only partly used to drive the low pressure compressor 5. A large amount of the mechanical power available on the low pressure turbine 11 output shaft 21 is used to drive the load.
The output shaft 21 of the power turbine or low pressure turbine 11 forms part of a load coupling 23, which transmits the mechanical power from the gas turbine 3 to a load. In the example of FIG. 1 the load is represented as a centrifugal compressor 25. A gear box 27 is provided in this exemplary embodiment between the gas turbine output shaft 21 and the centrifugal compressor 25. A gear box is usually provided when the rotary speed of the low pressure turbine on the one hand and of the load on the other is not identical in terms of rpm or if it has to be reversed. In some embodiments the power turbine 11 can be directly connected to the load shaft, i.e. to the shaft of an electric generator or a turbomachinery, such as a centrifugal compressor.
In a twin shaft aeroderivative gas turbine as the one illustrated in FIG. 1, washing of the compressors 5 and 7 requires both the first (high pressure) and the second (low pressure) gas turbine shafts to be rotated. The former is rotated with the onboard starting motor of the gas turbine itself, the latter requires an external mover. Moreover, rotating the second gas turbine shaft requires high power input because the second gas turbine shaft is permanently mechanically connected to the load.
Gas turbines are also used as prime movers in power generation applications, wherein mechanical power available on the gas turbine output shaft is used to drive an electric generator. The electric generator converts mechanical power from the gas turbine into electric power. Single-shaft gas turbines are often used in power generation applications of this kind. The gas turbine comprises a compressor and a turbine, mechanically connected to one another by a shaft. Compressed air provided by the compressor is delivered to a combustor and mixed with fuel therein. The air-fuel mixture is ignited to produce compressed hot combustion gases. The combustion gases are expanded in the gas turbine to generate mechanical power. Part of the mechanical power produced by the turbine is used to drive the compressor. Excess mechanical power is available on the single gas turbine shaft for driving the electric generator.
In order to start the single-shaft gas turbine and the electric generator mechanically linked thereto, the use of two combined movers is known. A first mover comprises a low speed electric motor. A second mover comprises a high speed, internal combustion engine. To start rotation of the gas turbine and generator train, the low speed electric motor is energized first. Once a pre-set rotary speed of the shaft line has been achieved, further acceleration of the system is performed by the high speed, internal combustion engine. The slow speed electric motor is also used for slow turning the power plant following shut down, to prevent bowing of the rotor aggregate of the gas turbine.